The present invention pertains in general to methods and apparatus for determining vapor pressure and pertains in particular to methods and apparatus for determining the Reid Vapor Pressure (RVP) of a volatile, liquid hydrocarbon.
In internal combustion engines, fuel is transported in liquid form but burned in gaseous form. If the fuel vaporizes while it is being transported in fuel pumps, lines or carburetors, fuel flow may be decreased to the point that power loss, rough engine operation or engine stoppage results. A fuel which does not vaporize readily enough may cause hard starting, poor warm-up, poor acceleration, and unequal fuel distribution among a number of cylinders. It is therfore desirable to obtain a fuel having a characteristic volatility which provides optimal engine performance for a given ambient temperature and pressure.
One measure of volatility is the RVP. Criteria for RVP tests and apparatus have been established by the American Society for Testing Materials (ASTM) in specifications designated D-323. State regulations require that automotive gasoline meet standards established for RVP. It is therefore important for refiners of automotive gasolines to have apparatus for detemining RVP.
An invention of the present inventors which duplicates all of the essential conditions of determining ASTM D-323, and the only automatic apparatus known by the present inventors to do so, is disclosed in U.S. Pat. No. 4,332,159, and is assigned to the assignee of the present invention. The disclosure of U.S. Pat. No. 4,332,159 is hereby incorporated by reference herein.
Other than the device disclosed in U.S. Pat. No. 4,332,159, automated devices known to the present inventors deviate from ASTM D-323 to the extent that their results must be correlated with RVP. Correlation error inherent in such results makes them less likely to be accurate than results obtained by direct measurement. Thus, a particular advantage of the apparatus of U.S. Pat. No. 4,332,159 is the fidelity with which RVP is measured. However, seals, such as those located around pistons used in the apparatus in U.S. Pat. No. 4,332,159 are subject to leakage and consequently reduce reliability.